Even though FSF 6.0.0 is still in development, it's entered stage 3
which is unusally relatively stable for the Ada front-end. It has
been placed in FreeBSD ports at lang/gcc6-aux, and the Ada framework
can optionally use it instead of GNAT FSF 5.3.

It currently builds everything except OpenToken (caught by new code
restriction / detection and needs code to be fixed) and Ironsides
which fails during compilation with a STORAGE error, so an internal
compiler error is suspected.

If you have FreeBSD or DragonFly and you want to testdrive GNAT 6,
then add ADA_DEFAULT=6 to /etc/make.conf and rebuild everything from
source.